The internal organization of bacterial cells is critical for many complex processes. For example, bacteria are able to survive harsh conditions or infect host cells through sporulation and secretion of virulence factors. Sporulation and in many cases virulence factor secretion require subcellular localization of proteins. The use of fluorescent tags has shown that proteins can have dynamic localization patterns within the cell, but the role of subcellular localization in the modulation of regulatory protein activity is still unclear. Histidine kinases are a major class of bacterial regulatory proteins that act in the signaling pathways responding to both internal and external signals. Using Caulobacter crescentus as a model system for the regulation of cell differentiation, this study seeks to determine how a histidine kinase and its putative localization factor help regulate the development of one of the offspring resulting from asymmetric division. The aims of this study are to: 1) Determine the relationship between a histidine kinase that modulates development and a known developmental regulator that is a major target for the polar localization of proteins within the cell. This will be accomplished by determining if they are in the same regulatory pathway through epistasis analysis, if these two proteins interact and are required for each other's polar localization, and how global gene expression is affected by single or double mutants in their genes. 2) Determine the role of the two proteins in the regulation of motility. This will be accomplished by determining the role of the kinase activity of the histidine kinase and the impact of the localization factor on this activity and by the identification and study of genes in the same regulatory pathway. Together these experiments will help understand how subcellular localization can modulate the activity of regulatory proteins.